1. Field of the Invention
The present invention relates generally to servo control systems used for positioning read/write transducers in data storage devices, and more particularly, to providing linearity compensation for a position error signal (PES) based on run out in a disk drive.
2. Description of Related Art
It is well known in the art to store data on magnetic or optical disk drives. Data is stored on a disk drive on one or more tracks of predetermined format disposed on a disk-shaped recording media. The data is written to and read from the tracks using one or more transducers, which typically comprise read/write heads. Reading data from a desired one of the tracks on the disk surfaces requires knowledge of the read/write head position relative to the track as the disk rotates and the head is moved across the disk, and requires precise centering of the head over the disk track. Conventionally, the read/write head is mounted on a head positioning assembly that is moved by a servo control loop.
The servo control loop controls movement of the head positioning assembly across the disk surface to move the read/write head from track to track (track seeking) and, once over a selected track, to maintain the read/write head in a path over the centerline of the track (track following). Centering the read/write head over a track permits accurate reading and recording of data in the track.
In most devices, the servo control loop is a closed loop system that utilizes position information obtained from the disk surface to provide feedback for the track seeking and track following functions. Some devices store servo information on a single, dedicated disk surface (known as a dedicated servo system), while other devices store servo information between the data regions of the disk surface (known as an embedded servo system).
Servo track information usually includes: a synchronization field, such as for automatic gain control (AGC) or similar signal detecting purposes; a track identification TID) field typically comprising a digitally encoded Grey code; and a position error signal (PES) field generally containing one or more burst patterns. The PES, which is proportional to the relative difference of the positions of the center of the read/write head and the nearest track centerline, is a corrective signal providing an indication of which direction the head should be moved to during either track seeking or track following functions.
As the track densities of disk drives increase, the accuracy of the PES becomes increasingly important. Unfortunately as track pitch decreases, the tolerances arising in the manufacture of the read/write heads do not decrease proportionately, and this results in heads that either write or read more narrowly relative to track pitch. This narrow read or write characteristic results in the PES signal varying in a non-linear fashion with the head position.
To minimize the limitations in the prior art described above, and to minimize other limitations that will become apparent upon reading and understanding the present specification, the present invention provides linearity compensation for a position error signal based on run out in a data storage device. The run out is measured on a track of the data storage device at a plurality of points, such as a track center and one or more points offset from the track center. The run out is typically measured using either an unstitched primary position error signal or an unstitched quadrature position error signal. The offsets are usually stitch points for the position error signal. A component of the run out is extracted at the points, wherein the component generally comprises at least one characteristic of the run out, such as a frequency, a set of frequencies, a root mean square, or some other characteristic of the run out. The linearizing equation to be used for the position error signal is identified based on the measured run out and generally comprises a polynomial function. The coefficients of the linearizing equation are adjusted so that the run out and the component of the run out are substantially similar at the measured points. These steps are repeated until a satisfactory fit is achieved in the linearizing equation. Once a satisfactory fit is achieved, the coefficients for the linearizing equation are stored in the data storage device, so that they can be accessed by a servo controller during operation of the data storage device. The servo controller uses the linearizing equation to generate a linearized position error signal that is used to improve a track following capability of the data storage device.